An independent power source type charging system including a power generating unit using natural energy such as sunlight or wind, a cartridge type (removable) secondary battery pack being charged with power generated by the power generating unit, an electric power load such as an illumination lamp or a display used in the charging system, and a control unit controlling the operation of the whole charging system has been proposed. As described above, it is preferable that the independent power source type charging system can operate independently without any supply of power from another place by controlling the operation of the whole charging system by the use of power generated by the power generating unit using natural energy. However, since the power generating unit based on natural energy generation varies in power generation depending on presence of wind, weather, or time zone, the supply of power is not stable and the operation control of the control unit is affected, thereby destabilizing the operation of the whole charging system. For example, at night and when there is no wind, the power generation is disabled in both solar power generation and wind power generation, thereby disabling the operation control of the electric power load.
A technique of charging a secondary battery pack is disclosed in Patent Document 1.